This invention relates to a method of measuring the change in distance between a datum point and a body in relative movement away from that datum point, and to an instrument for performing that method.
Provided that the medium between the datum point and the moving body is not too heavily attenuating, such measurements can in principle be made using either `time of flight` or `doppler` type radar measuring principles. However, for some applications adequate transparency of the intervening medium cannot be relied upon.